Two-stage pressure systems have been proposed for many applications. One particular application is the use of pressure systems as welding clamps, for the spot welding of metals.
There are numerous other applications are available for a two-stage type pressure cylinder.
One form of two-stage clamping cylinder has been proposed in U.S. Pat. Nos. 3,875,365, 3,828,652, 4,099,436 and 4,135,076.
In this system the hydraulic fluid is located in a holding tank in a separate location, and is connected by hoses to a work cylinder.
However this system does have certain disadvantages. Each work cylinder must be provided with its own hydraulic fluid tank and system of hoses, and the tank must be capable of receiving a pressure from an air pressure source.
French Patent 2,384,977 shows a two-stage air-hydraulic cylinder. The cylinder has a work piston, an oil piston and an intensifier piston.
However, the operating system for the cylinder is a complex system of air chambers, controlled by needle valve at the top of the cylinder on the left hand side of FIG. 1. The intensifier piston 26 must be all the way to the left in order for these needle valves to operate. So long as the intensifier piston 26 is kept at the left hand end, then oil can flow from the oil cylinder 21 through the chamber 13 into the working cylinder.
However, if there is any malfunction of the needle valves caused by dirt or any other means, so that the intensifier piston 26 does not reach all the way back to the left, then no oil flow can take place since the cylinder rod 15 will close off the passage 13.
There is no disclosure of a first valve allowing oil to flow into the working cylinder on a working stroke and preventing its return flow.
There is no disclosure of a second valve which is normally closed to prevent return flow, and which is operable to permit return flow, for a return stroke of the working piston.
German Patent 3,345,002 discloses a two-stage cylinder. There are only two pistons namely, a work piston and an intensifier piston. Oil is apparently contained in the work cylinder, and is in some way forced upwardly by means of air in the conduit 19 and the spring 13. Apparently, this movement will cause the initial downward movement of the work piston until it touches the workpiece.
There is no disclosure of a first valve normally operable to permit flow of oil from an oil cylinder into the working cylinder, and being closed to prevent reverse flow.
There is no disclosure of a valve which is normally closed to prevent reverse flow, but which is operable to open and permit reverse flow of the oil for a return stroke of the work piston.